In general, after manufacture, the platters of a hard disk drive need to be tested for defects and to ensure that they meet specifications. Testing is typically performed on unformatted disk platters prior to final disk drive assembly. Small scratches, pits and other defects in the surface of the magnetic film are particularly critical and the existence of any such defects needs to be identified. By identifying the location of spatial defects, data loss is avoided by marking the area as defective prior to use, or by discarding the disk entirely if it is found to have too many defects.
In addition to spatial defects such as scratches or other irregularities in the disk surface, hard disks are also subject to “thermal” defects that may occur along with or separately from the spatial defects. Magnetic media storage devices such as hard disk drives suffer from recorded signal damage when a particle trapped under the read/write head of the hard disk drive causes enough friction to create a thermal event that changes the magnetic state of the of the recorded signal. For example, this type of defect is essentially a small bump or protrusion on the surface of the platter, including the surfaces of the hard disk, where the height of the bump is such that the read or write head makes contact with the bump, but is able to continue scanning the surface of the disk (i.e., the bump is not so large that the read head stops functioning). When the read head encounters the bump, the high speed impact causes the read head to increase in temperature (hence the name “thermal” defect). Repeated impacts lead to wear on the read head and can eventually cause the head to “crash” into the surface of the hard disk. As such, thermal defects on a hard disk pose an even greater problem than spatial ones. While spatial defects may limit the amount of disk space available to store data, thermal defects may cause the hard disk to crash, such that data on the disk may or may not be recoverable. For this reason, a disk having predominantly spatial defects and few thermal ones will be more usable than a disk having the same total number of defects, but where a substantial number of the defects are thermal.
Accordingly, there is a need for a system and method to identify spatial and thermal defects and other defective characteristics on a magnetic media surface and to distinguish each type of defect from the other.